Why Lord Dredmor is trapped in the bottom of the dungeon

He’s spending all his time trying to research a spell to open doors, but alas, spells cannot effect doors. And none of the monsters he summons into his dungeon to help him out can, either.
Why can’t he or any of the monsters open doors?

Well, long ago, the diggle Archarchmages (the secret leaders of the secret leaders of the secret leaders of diggle society) sought ULTIMATE POWER. So they tried to dig all the way to diggle paradise, which was, of course, at the bottom of the world.

When they finally arrived, they found, blocking their way, a great door. (“Acquire Hope, All Ye Who Enter Here”) But they were diggles, so they just dug through the door.

Whereupon the Diggle Maker saw this blasphemy, and was very cross, saying “And so Diggle Paradise is blackened with each step you take in my hall! Marvel at perfection for it is fleeting. You have brought sin to Paradise, and doom to all the underworld!” And thus the Diggle ArchArchmages were cast out, twisted and cursed by their own corruption, adn returned as monsters, the first of the diggle darkspawn. Except their leader, Vlad Digula, and most of their minions, who remained trapped in what was now Diggle Hell.

But more significant was the offense the Diggle God of Doors took at the ruining of such a fine door. The Diggle God of Doors turned its back on digglekind, forsaking digglekind, cursing them, and all other monsters of the underworld to never be able to open a door again. (The Diggle God of Doors also re-endoored Diggle Hell with a door with a more suitable maxim for hell.)

Not that this mattered much to the diggles, for the Diggle god of Digging just taught them to dig AROUND doors. So Diggles can still freely navigate the dungeon through secret DiggleTunnels (tm).

And so, when Dredmor shed the bonds of mortality, becoming a lich, and thus, a monster, he found himself trapped in the bottom of his dungeon lab, since he, unlike the diggles, couldn’t dig his way out.

…and you’ve been fighting your way through the dungeon, breaking down doors with your glorious foot.